BELFAST -- The first abortion clinic on the island of Ireland opened Thursday in downtown Belfast, unleashing angry protests on the street and uniting Catholic and Protestant politicians in calls to investigate the new facility.

The clinic, run by the British family planning charity Marie Stopes, will be permitted to provide abortions only in exceptional circumstances to women less than nine weeks pregnant.

But the opening caught Northern Ireland's socially conservative politicians off guard, and they vowed to launch an investigation into how the clinic operates. About 400 protesters who lined the sidewalk outside the facility all day said they were certain that public pressure would force authorities to shut it.

"I expect the heads of government to run Marie Stopes out of Northern Ireland," the protest leader, Bernadette Smyth of the pressure group Precious Life, told supporters through a bullhorn. "Those who have come ... storm heaven with your prayers!"

Abortion is one of few issues that unites Northern Ireland, a predominantly Protestant corner of the United Kingdom, and the mostly Catholic Republic of Ireland. Both jurisdictions keep abortion outlawed except in cases where doctors deem the woman's life at risk from continued pregnancy.

Both effectively export the controversy to Britain, where abortion on demand has been legal since 1967. An estimated 4,000 women from the Irish Republic, and 1,000 from Northern Ireland, travel there for abortions annually, often lying to family, friends and colleagues about their absence.

Inside the clinic on Thursday, doctors and counsellors dealt with several women in crisis pregnancies. They reported being deluged with calls from women, including Republic of Ireland residents, seeking appointments.

Outside, protesters displayed posters with graphic pictures of aborted fetuses, sang hymns and sparred verbally with passing pedestrians, who made clear they want liberalized access to abortion in Northern Ireland. Protesters didn't directly heckle people entering or leaving the clinic, which is inside a much larger building with several offices.

Directors of Marie Stopes emphasized they would comply fully with Northern Ireland's law permitting abortions only when the woman's life or long-term health is endangered. They said while such exceptional abortions are already carried out in secrecy in Northern Ireland hospitals, between 30 and 50 a year, many more eligible women travel to Britain rather than confront stern anti-abortion attitudes at home.

Tracey McNeill, director of Marie Stopes clinics across the United Kingdom, said some of the approximately 1,000 women who travel each year to Britain for abortions "would have been entitled to have that care within Northern Ireland, but they didn't know where to go, they didn't know who to talk to."

The Belfast clinic, she said, "is not about increasing the number of terminations of pregnancies in Northern Ireland. It's about providing it to that small number of people who will be eligible for it within their own country."

The senior legal adviser to Northern Ireland's Catholic-Protestant government, Attorney General John Larkin, said he would be happy to aid any legislative investigation into the clinic. Lawmakers quickly accepted the suggestion and said they would summon clinic officials to fact-finding hearings, with Larkin free to ask questions, too.

"Given the contentious nature of their support for abortion, it is necessary that the law is fully complied with and that we are assured by Marie Stopes," said Alban Maginness, a Catholic member of the legislature's justice committee.

"There is huge public interest. It's only appropriate to examine (the clinic). The public expect us to do something," said Jim Wells, a Protestant member of the health committee.

Clinic directors say the only form of abortion they will provide are pills that induce miscarriages in women up to nine weeks pregnant. Such pills are already easily ordered from British suppliers on the Internet, though receipt of such pills in Ireland could be treated as a criminal offence.

Suzanne Lee, a 23-year-old student from Northern Ireland who had a pill-induced abortion last year in Dublin after ordering it off the Internet, said she would have liked to be able to go to a Marie Stopes clinic for medical support, because taking the medication was "quite an ordeal to go through."

She said it involved "severe cramping, a lot of bleeding. I bled for four weeks after it, but because I terminated my pregnancy at six weeks, it was nothing worse than a very bad period." She expressed disgust that many people in Northern Ireland "believe I should spend life in prison for what I did."

Protesters warned that the clinic, if not closed, would become a beachhead for expanding abortion rights in Northern Ireland and, eventually, the Republic of Ireland.

"For Marie Stopes, this is only a first step," said Liam Gibson from the Society for the Protection of Unborn Children, a predominantly Catholic pressure group.

The Roman Catholic Church in Ireland this week launched a monthlong campaign to press the Irish government to strengthen its constitutional ban on abortion. It has denounced the Belfast clinic's opening but shied away from calling for protests.

"We are in the middle of a struggle for the soul of Northern Ireland," said Bishop Donal McKeown, the senior Catholic in Belfast, who didn't attend the protest. He said directors of Marie Stopes were seeking "to promote the acceptability of abortion."

Only one of Northern Ireland's 108 legislators, Anna Lo, has expressed support for the clinic.

While opinion polls indicate public opinion is split roughly 50-50 on the issue, taking a pro-choice stand is seen as a vote-loser. As a result, Northern Ireland has failed to produce legally binding guidelines for doctors explaining the precise circumstances when abortions can be performed legally here.

Doctors and nurses have asked repeatedly for clearly written government rules to guard them from protests or lawsuits if they're identified as abortion providers. This inaction means that the only legislation dates to 1861, outlawing the "procurement of a miscarriage," and a 1945 amendment creating the exception that permits abortions to preserve the mother's life or health.

The law is even messier in the Republic of Ireland, which won independence from Britain in 1922.

Its constitution bans abortion, but in 1992 the Irish Supreme Court ruled it was legal to receive abortions there, if the woman's life was in danger -- including from her own threats to commit suicide if denied one. Successive governments have refused to pass legislation in line with that judgment.